The 3D printing wars are hotting up, as Thingiverse users upload a design for an AR-15 magazine (holds five rounds, easy to expand to as many rounds as the material will hold without snapping) and an AR-15 lower receiver, a component whose ownership is proscribed or regulated in most of the USA.

This is a near fully printable 5.56mm X 45mm or .233 REM AR-15 magazine. It is current only a 5 round magazine. I left my printed spring design out on purpose for saftey reasons. However, with a little printing experimention and some range time it can be made easily.

The Lower Receiver is the frame that holds together all the other pieces of the firearm. In the States, all the other pieces can be purchased without a permit - over the counter or through the post. The Lower Receiver is the only part which requires a background check or any other kind of paperwork before purchase.
Typically this part is made of aluminium. A rifle with a Lower Receiver made of plastic can be perfectly functional.